Ecology and diversity of indigenous Trichoderma species in vegetable cropping systems

Abstract

The overall aim of this research was to improve the understanding of the ecology and
diversity of Trichoderma species within the soil and rhizosphere of onion (Allium cepa
L.) and potato (Solanum tuberosum L.) under intensive management in New
Zealand.
The indigenous Trichoderma population was measured in a field trial at Pukekohe
over a three year period under six different crop rotation treatments. The treatments
included two continuous onion and potato rotations (intensive), two onion/potato
mixed rotation (conventional), and two green manure rotations (sustainable). Results
showed that Trichoderma populations were stable in both the rhizosphere and bulk
soil (1.5 x 10² to 8.5 x 10³ CFU g⁻¹ ODS). The planting and incorporation of an oat
(Avena sativa L.) green manure in the sustainable rotations positively increased
Trichoderma colony forming unit (CFU) numbers in the rhizosphere soil from 3.4 x
10² to 2.5 x 10³ g⁻¹ ODS. A Trichoderma species identification method was developed
based on colony morphology. Representative isolates were verified using restriction
fragment length polymorphism (RFLP) and DNA sequencing. The method allowed for
rapid and reliable identification of isolated Trichoderma species. Five species were
identified in the Pukekohe soil: T. asperellum, T. atroviride, T. hamatum, T.
harzianum and T. koningii. Results showed identical species diversity between the
rhizosphere, rhizoplane and bulk soil. The species did not strongly compete between
each other for the rhizosphere ecological niche and differences in species
proportions seemed to be caused by environmental factors rather than the rotation
treatments.
The incorporation of oat green manure in pots did not significantly promote the
indigenous Trichoderma population size and diversity in the rhizosphere of onion
plants up to 4 months old. The identified species were the same as in the field trial.
The incorporation of onion scale residues was shown to result in low Trichoderma
and high Penicillium CFU numbers and a reduction in plant size. Additionally, the
presence of high levels (6.0 x 10⁵ CFU g⁻¹ ODS) of Penicillium CFU was negatively
correlated with the presence of Trichoderma CFU. The effect of oat incorporation on
Trichoderma saprophytic growth was also investigated in a soil sandwich assay and
revealed no significant differences.
A series of experiments indicated that onion extract obtained from dry onion scale
residues had no antifungal activity against either Trichoderma or Penicillium and
instead tended to promote their hyphal growth and sporulation. It also showed that
competition between Penicillium and Trichoderma isolates was limited despite the
ability of Penicillium to produce a wide range of inhibitory substances.
Four indigenous Trichoderma species (T. atroviride, T. hamatum, T. harzianum and
T. koningii) were shown to be rhizosphere competent in a split tube experiment over
a 6 week period. The results of this experiment revealed that, the Trichoderma
species clearly displayed differences in their ability to colonise the rhizosphere of
young onion seedlings. Species such as T. koningii had the greatest rhizosphere
colonising ability regardless of soil depth while T. harzianum displayed the weakest
ability. Results also indicated that when inoculated as a mixture the four species
competed with one another to colonise the rhizosphere.
Overall, this research indicated that the studied crop rotation treatments and the use
of oat as a green manure did not strongly promote indigenous Trichoderma
populations. Species diversity was constant throughout the research with T.
hamatum and T. koningii being the most frequently isolated species.... [Show full abstract]